It is known that active carbon alone or impregnated with iodine and/or iodide can be used for the removal of organic iodine compounds and/or iodine from gases and more especially for the separation of radioactive fission product iodine from atmospheres in nuclear plants [R. E. Adams, R. D. Ackley; Chapter 2.1: "Trapping of Radioactive Iodine and Methyliodide by Iodized Charcoal" in Nuclear Safety Program Annual Progress Report for Period Ending Dec. 31, 1967 ORNL-4228 (April, 1968, pp. 99 to 114)]. Amine-impregnated active carbons are also used.
However, this adsorption material cannot be used universally because it is flammable and releases the adsorbed iodine again in significant quantities at relatively low temperatures, for example 150.degree. C. If relatively high temperatures occur in the gas to be cleaned or if the adsorber material is likely to be strongly heated by the heat of decay of radioactive fission products, temperature-resistant and non-flammable materials must be used.
It has been found that fission product iodine occurs in waste gases of nuclear plants not only in elemental form, but also in the form of organic compounds containing a small number of carbon atoms, for example in the form of radioactive methyl iodide. For this reason, adsorber materials, which are supposed to be generally useable, also have to retain organic iodine compounds in equal measure.
Other adsorber materials which may be used for the separation of iodine, possibly under precisely defined adsorption conditions, such as for example silver-impregnated ceramic sintered products (so-called Berl saddles), silver-coated copper chips or silver-coated silica gel, are either insufficiently effective or totally ineffective for retaining methyl iodide or lose their effectiveness when superheated steam is passed through. In addition, impregnated silica gel has the property of taking up water and thus losing its strength. Accordingly, these materials are unsuitable for general application, i.e. for use in various iodine removal plants under possibly different or even rapidly changing adsorption conditions, for example during or after an accident.
Although the shaped silver-impregnated sorbent particles described in DE-OS 21 09 146, which consist predominantly of amorphous silica, are capable of effectively sorbing iodine and iodine compounds and are unaffected by superheated steam, they have the disadvantage that the salt impregnation can be washed out under the superheated steam conditions at 150.degree. C.
By contrast, silver-exchanged molecular sieve zeolites are resistant to washing out. Zeolites are alumosilicate structures having the following general formula EQU M.sub.m/z [mAlO.sub.2.n SiO.sub.2 ]. q H.sub.2 O
in which M.sub.m/z are exchangeable cations, [mAlO.sub.2.nSiO.sub.2 ] is the anionic framework and q H.sub.2 O is the sorbed phase. Corresponding zeolites are described, for example, in D. W. Breck, Zeolite Molecular Sieves, John Wiley & Sons, Inc., New York, 1974.
Silver-exchanged molecular sieves have already been investigated for the sorption of iodine [D. T. Pence, F. A. Duce, W. J. Maeck, Proceedings 12th AEC, Air Cleaning Conference, Oak Ridge, Tenn., January 1973, p. 417; J. G. Wilhelm: "Trapping of Fission Product Iodine with Silver Impregnated Molecular Sieves", presented at the International Congress of the Diffusion of Fission Products, Saclay, France, Nov. 4th to 6th, 1969; Report of the Gesellschaft fur Kernforschung mbH, Karlsruhe, No. KFK-1065 (October 1969)]. Suitable molecular sieves are sodium alumosilicates, for example having a composition represented by the following summation formula EQU Na.sub.86 [AlO.sub.2).sub.86 (SiO.sub.2).sub.106 ]..times.H.sub.2 O
with the structure of faujasite.
In the treatment with silver nitrate, the sodium ions are exchanged for silver ions. High degrees of separation are achieved for methyl iodide and elemental iodine at high relative air humidities. According to the prior art, clay-bound silver-exchanged zeolite granulates are used for the adsorption of iodine. Degrees of separation of 99.9% can be achieved in this way. In order to reduce the high cost of the iodine sorption filters, it is desirable to achieve higher degrees of separation.